Porous support, preparation method therefor, and reinforced membrane containing same

ABSTRACT

The present invention relates to a porous support, a method for manufacturing the same, and a reinforced membrane comprising the same, the porous support comprising a nanoweb in which nanofibers are accumulated in the form of a nonwoven fabric including a plurality of pores, wherein the nanoweb has a moisture saturation time of 1 second to 600 seconds. The porous support not only has excellent durability, heat resistance, and chemical resistance while exhibiting excellent air permeability and water permeability, but also has good hydrophilicity.

TECHNICAL FIELD

The present invention relates to a porous support, a method of manufacturing the same, and a reinforced membrane comprising the same. More particularly, the present invention relates to a porous support which exhibits superior gas permeability and water permeability, excellent durability, heat resistance and chemical resistance, and superior hydrophilicity, a method of manufacturing the same, and a reinforced membrane comprising the same.

BACKGROUND ART

Nanofibers are used in a variety of applications such as filters for water purification, filters for air purification, composites, membranes for cells and the like, in particular, reinforced composite membranes for fuel cells for cars, due to wide surface area and excellent porosity.

A fuel cell is an electrochemical device which is operated from hydrogen and oxygen as fuels, which arises as an environmentally friendly device because its products are pure water and recyclable heat. In addition, it is widely used as power sources for household, car and power generation applications and the like owing to advantages such as easy operation, high output density and non-noise.

Depending on the type of electrolyte membrane, the fuel cell is classified into an alkaline electrolyte fuel cell, a direct oxidation fuel cell, a polymer electrolyte membrane fuel cell (PEMFC) and the like. Of these, the polymer electrolyte membrane fuel cell generates electricity based on transfer of hydrogen ions (H⁺) from an oxidation electrode (anode) to a reduction electrode (cathode), which can operate at room temperature (20° C.) and have an advantage of short activation time, as compared to other fuel cells.

A polymer electrolyte membrane fuel cell includes an electricity generator which includes a membrane electrode assembly (MEA) which is provided with an oxidation electrode and a reduction electrode which are opposite to each other based on a polymer electrolyte membrane fuel cell interposed therebetween, and a separator (also, referred to as a “bipolar plate”), a fuel supply to supply a fuel to the electricity generator, and an oxidizing agent supply to supply an oxidizing agent such as oxygen or air to the electricity generator.

A polymer electrolyte membrane is a conductor of hydrogen ions and may be classified into a single membrane including a polymer such as a fluorine- or hydrocarbon-based polymer and a composite membrane including a composite of the polymer with an organic/inorganic substance, a porous support or the like. The most generally used single membrane is Nafion available from DuPont which is a perfluorine-based polymer. However, Nafion has drawbacks of high price, low mechanical and shape stability, and high membrane resistance due to high thickness.

In order to solve these drawbacks, a research is underway on composite membranes with reinforced mechanical and shape stability. Of composite membranes, a pore-filling membrane including a porous support impregnated with an ion conductor is actively researched due to low price as well as excellent performance and mechanical shape stability.

The support generally used for pore-filling membranes is polytetrafluoroethylene (PTFE). However, a PTFE support has superior chemical resistance, but has a drawback of low porosity of 40 to 60%.

PRIOR ART DOCUMENT Patent Document

-   1) Korean Patent Laid-open Publication No. 2011-0120185 (published     on Nov. 3, 2011)

DISCLOSURE Technical Problem

Therefore, it is an object of the present invention to provide a porous support which exhibits superior gas permeability and water permeability, excellent durability, heat resistance and chemical resistance, and superior hydrophilicity.

It is another object of the present invention to provide a method of manufacturing the porous support.

It is another object of the present invention to provide a reinforced membrane comprising the porous support.

Technical Solution

In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a porous support including a nanoweb in which nanofibers are integrated in the form of a non-woven fabric including a plurality of pores, wherein the nanoweb has a moisture content saturation time of 1 sec to 600 sec.

The nanoweb may have a moisture regain of 3.0% by weight or more.

The nanoweb may have wettability in accordance with wicking test, of 2 to 15 cm.

The nanoweb may have a contact angle of 90° or less.

The nanofiber may include 0.1 to 20 parts by weight of a polymer hydrophilic additive, with respect to 100 parts by weight of the nanofiber.

A hydrophilic additive may be impregnated in the pores of the nanoweb.

A hydrophilic additive may be coated on one or two surfaces of the nanoweb.

The hydrophilic additive may be selected from the group consisting of TiO₂ anatase, TiO₂ rutile, TiO₂ brookite, tin dioxide (SnO), zirconium dioxide (ZrO₂), aluminium oxide (Al₂O₃), oxidized single-walled carbon nanotubes, oxidized multiwalled carbon nanotubes, graphite oxide, graphene oxide and a combination thereof.

The hydrophilic additive may be selected from the group consisting of polyhydroxyethylmethacrylate, polyvinylacetate, polyurethane, polydimethylsiloxane, polyimide, polyamide, polyethyleneterephthalate, polymethylmethacrylate, epoxy and a combination thereof.

The hydrophilic additive may have a mean diameter of 0.005 to 1 μm.

The nanofiber may include a polyimide nanofiber.

The polyimide may have a main chain including a substituent selected from the group consisting of an amine group, a carboxyl group, a hydroxyl group and a combination thereof.

The polyimide may be prepared by polymerizing diamine, dianhydride and a comonomer containing a hydroxyl group to prepare polyamic acid and then imidizing the polyamic acid.

The comonomer containing a hydroxyl group may be selected from the group consisting of dianiline containing a hydroxyl group, diphenyl urea containing a hydroxyl group, diamine containing a hydroxyl group and a combination thereof.

One or two surfaces of the nanoweb may be plasma-treated.

An inorganic substance may be deposited on one or two surfaces of the nanoweb.

The inorganic substance may be selected from the group consisting of TiO₂ anatase, TiO₂ rutile, TiO₂ brookite, tin dioxide (SnO), zirconium dioxide (ZrO₂), aluminium oxide (Al₂O₃), oxidized single-walled carbon nanotubes, oxidized multiwalled carbon nanotubes, graphite oxide, graphene oxide and a combination thereof.

In another aspect of the present invention, provided is a method of producing a porous support including adding diamine and dianhydride to a solvent to prepare an electrospinning solution, electrospinning the prepared electrospinning solution to produce a polyamic acid nanoweb including nanofibers integrated in the form of a non-woven fabric including a plurality of pores, and imidizing the polyamic acid nanoweb to produce a polyimide nanoweb, wherein the polyimide nanoweb has a moisture content saturation time of 1 sec to 600 sec.

A comonomer containing a hydroxyl group may be further added to the electrospinning solution.

The nanofibers present in the polyimide nanoweb may have a main chain substituted by a substituent selected from the group consisting of an amine group, a carboxyl group, a hydroxyl group and a combination thereof.

In another aspect of the present invention, provided is a method of producing a porous support including electrospinning an electrospinning solution to produce a nanoweb in which nanofibers are integrated in the form of a non-woven fabric including a plurality of pores,

wherein the nanoweb has a moisture content saturation time of 1 sec to 600 sec.

A hydrophilic additive may be further added to the electrospinning solution.

The method may further include impregnating a hydrophilic additive in pores of the nanoweb.

The method may further include coating a hydrophilic additive on one or two surfaces of the nanoweb.

The method may further include plasma-treating one or two surfaces of the nanoweb.

The plasma treatment may be carried out by treating one or two surfaces of the nanoweb with gas for imparting a hydrophilic group using low-temperature plasma or radio frequency (RF) plasma.

The method may further include depositing an inorganic substance on one or two surfaces of the nanoweb.

The deposition of the inorganic substance may be carried out by sputtering.

In another aspect of the present invention, provided is a reinforced membrane including the porous support and an ion exchange polymer filling pores of the porous support.

Other details of embodiments of the present invention are incorporated in the Detailed Description of the Invention described below.

Effects of the Invention

The porous support according to the present invention exhibits superior gas permeability and water permeability, excellent durability, heat resistance and chemical resistance, and superior hydrophilicity.

DESCRIPTION OF DRAWINGS

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view illustrating a nozzle-type electrospinning device.

BEST MODE

Hereinafter, embodiments of the present invention will be described in detail. These embodiments are provided only as examples and should not be construed as limiting the scope and spirit of the present invention. The present invention is defined only by the scope of claims given later.

The terms used herein are used merely to describe specific embodiments, but are not intended to limit the present invention. The singular expressions include plural expressions unless explicitly stated otherwise in the context thereof. It should be appropriated that in this application, the terms “include(s),” “comprise(s)”, “including” and “comprising” are intended to denote the presence of the characteristics, numbers, steps, operations, elements, or components described herein, or combinations thereof, but do not exclude the probability of presence or addition of one or more other characteristics, numbers, steps, operations, elements, components, or combinations thereof.

As used herein, the term “nano” means a nano-scale and covers a size of 1 nm or less.

As used herein, the term “diameter” means a length of a short axis passing through a center of a fiber and the term “length” means a length of a long axis passing through the center of the fiber.

The porous support according to an embodiment of the present invention includes a nanoweb in which nanofibers are integrated in the form of a non-woven fabric including a plurality of pores.

The nanofibers preferably include a hydrocarbon-based polymer which exhibits superior chemical resistance and hydrophobicity and is thus free from shape deformation by moisture under high humidity environments. Specifically, the hydrocarbon-based polymer may be selected from the group consisting of nylon, polyimide, polyaramide, polyether imide, polyacrylonitrile, polyaniline, polyethylene oxide, polyethylene naphthalate, polybutylene terephthalate, styrene butadiene rubber, polystyrene, polyvinyl chloride, polyvinyl alcohol, polyvinylidene fluoride, polyvinyl butylene, polyurethane, polybenzoxazole, polybenzimidazole, polyamide-imide, polyethylene terephthalate, polyethylene, polypropylene, a copolymer thereof and a mixture thereof. Of these, polyimide which exhibits relatively better heat resistance, chemical resistance and shape stability is preferably used.

The porous support includes an assembly of nanofibers, in which the nanofibers produced by electrospinning are randomly arranged, that is, a nanoweb. The nanofibers preferably have a mean diameter of 40 to 5,000 nm, wherein the mean diameter is obtained as an average of the diameters of 50 nanofibers measured with a scanning electron microscope (JSM6700F, JEOL) in consideration of porosity and thickness of the nanoweb. When the mean diameter of the nanofibers is lower than 40 nm, mechanical strength of the porous support may be deteriorated and, when the mean diameter of the nanofibers exceeds 5,000 nm, porosity may be decreased and thickness may be increased.

The nanoweb includes the nanofibers described above, thereby having a porosity of 50% or more. As the porous support has a porosity of 50% or more, specific surface area of the porous support increases, thus facilitating impregnation of the ionic conductor upon application to a separation membrane and, as a result, improving efficiency of cells. Meanwhile, the nanoweb preferably has a porosity of 90% or less. When the porosity of the porous support exceeds 90%, subsequent processes may not be efficiently performed due to deterioration in shape stability. In addition, in another example, the porosity is calculated as a ratio of an air volume with respect to a total volume of the porous support in accordance with the following Equation 1. In this case, the total volume is calculated by producing a rectangular porous support sample and measuring the width, length and thickness of the sample, and the air volume is obtained by measuring a weight of the sample and subtracting a polymer volume, calculated back from polymer density, from the total volume.

Porosity (%)=(Air volume in porous support/Total volume of porous support)×100  [Equation 1]

In addition, the porous support may have a mean thickness of 5 to 40 μm. When the thickness of the porous support is less than 5 μm, mechanical strength and dimensional stability may be significantly deteriorated upon application to a separation membrane and, on the other hand, when the thickness exceeds 40 μm, resistance loss may increase upon application to the separation membrane and weight reduction and integration may be deteriorated. More preferably, the nanoweb may have a mean thickness of 10 to 30 μm.

In order that the nanoweb includes nanofibers having superior porosity and an optimal diameter, has a thickness, is easily produced and exhibits superior tensile strength after impregnation with an electrolyte, the polymer constituting the nanoweb preferably has a weight average molecular weight of 30,000 to 500,000 g/mol. When the weight average molecular weight of the polymer constituting the nanoweb is less than 30,000 g/mol, porosity and thickness of the nanoweb can be easily controlled, but porosity and tensile strength after wetting may be deteriorated. On the other hand, the weight average molecular weight of the polymer constituting the nanoweb exceeds 500,000 g/mol, heat resistance may be slightly improved, but the manufacture process does not smoothly proceed and porosity may be deteriorated.

In addition, as the nanoweb has a weight average molecular weight satisfying the range defined above and the polymer precursor is converted into the polymer under optimal curing conditions, it has heat resistance of 180° C. or more, preferably 300° C. or more. When the heat resistance of the nanoweb is less than 180° C., the nanoweb may be readily deformed at a high temperature due to degraded heat resistance and, as a result, performance of the electrochemical device produced using the same may be deteriorated. In addition, when heat resistance of the nanoweb is degraded, the nanoweb may be deformed by abnormal heating and performance thereof may be deteriorated, in serious cases, the nanoweb may be disadvantageously broken and exploded.

The nanoweb is insoluble in an organic solvent at room temperature (at 20° C.) to 100° C. and is thus chemically stable. The organic solvent may be an ordinary organic solvent such as NMP, DMF, DMAc, DMSO or THF.

The nanoweb may have a strain of 10 length % or less, preferably 5 length % or less. The strain may be obtained by standing a nanoweb sample with a width 100 mm and a length of 100 mm at 200° C. for 24 hours and calculating an average of width and length strains before and after standing. When the strain exceeds 10 length %, dimensional stability of the support may be deteriorated and shape deformation may occur under high temperature environments.

When the nanoweb includes polyimide, the imide conversion may be 90% or more, preferably 99% or more. The imide conversion may be measured by measuring an infrared spectrum of the nanoweb and calculating a ratio of imide C—N absorbance at 1,375 cm⁻¹ to a p-substituted C—H absorbance at 1,500 cm⁻¹. When the imide conversion is less than 90%, physical properties are deteriorated and shape stability cannot be secured due to un-reacted substance.

The nanoweb has air permeability of 50 to 250 μlpm, preferably 100 to 150 lpm. The air permeability may be measured in accordance with the method of ISO 9237. When the air permeability is less than 50 lpm, absorption of electrolyte may be difficult and, when the air permeability exceeds 250 lpm, the electrolyte may not be sufficiently incorporated.

The nanoweb exhibits superior hydrophilicity and thus a moisture content saturation time of 1 sec to 600 sec, preferably 1 sec to 300 sec, more preferably 1 sec to 180 sec, even more preferably 1 sec to 60 sec. The moisture content saturation time may be obtained from a time for which a sample is completely wet with water which is dropped from a height of 25 mm in accordance with KS K ISO 9073-6, textile-non-woven test method-Part VI: liquid absorption measurement method of absorption measurement standard.

When the moisture content saturation time is within the range, in the production of a reinforced membrane by impregnating an ion exchange polymer in the nanoweb, a great amount of the ion exchange polymer can be uniformly impregnated throughout pores of the nanoweb. In addition, as hydrophilicity of the nanoweb increases, when the reinforced membrane is used as a membrane for fuel cells, formation of hydrophilic channel is facilitated and ion conductivity can be thus improved.

The nanoweb has an electrolyte absorption capacity of to 60% by weight, preferably 30 to 40% by weight. In accordance with KS K ISO 9073-6, textile-non-woven test method-Part VI: liquid absorption measurement method of absorption measurement standard, the electrolyte absorption capacity may be measured by dropping a 70/30 (v/v) mixture of ethyl methyl carbonate and ethylene carbonate from a height of 25 mm for 60 sec, vertically draining the mixture for 120 sec, measuring the weight of the nanoweb and performing calculation in accordance with the following Equation 2. When the electrolyte absorption capacity is less than 10% by weight, performance of cells cannot be sufficiently obtained due to poor electrolyte absorption and, when the electrolyte absorption capacity exceeds 60% by weight, physical properties of the support may be deteriorated.

Electrolyte absorption capacity (%)=(W1−W)/W×100  [Equation 2]

wherein W is weight of nanoweb before absorption of electrolyte and W1 is weight of nanoweb after absorption of electrolyte.

The nanoweb may have a moisture regain of 3.0% by weight or more, preferably 3.0 to 5.0% by weight, more preferably 3.1 to 5.0% by weight. The moisture regain may be obtained by measuring, in accordance with KS K 0221, method of absorbing moisture of textile: oven balance method, the weight of a sample (O) after the sample reaches moisture equilibrium for 24 hours under laboratory standard conditions (KS K 0901), measuring the weight of the dried sample (D) at 105 to 110° C. for 90 minutes and performing calculation in accordance with the following Equation 3:

Moisture regain (% by weight)=(O−D)/D×100  [Equation 3]

(O: weight of sample, D: weight of dried sample)

The nanoweb may have a wettability obtained in accordance with the wicking test, of 2 to 15 cm, preferably 2.1 to 15 cm, more preferably 3 to 15 cm. The wicking test may be carried out by immersing a sample for 30 minutes and then measuring a wicking maximum distance in accordance with USA AATCC Test Method 197-2011, Option B, Measure distance at a given time in Vertical Wicking of Textiles. When the wettability in accordance with the wicking test is less than 2 cm, a problem in which an ion conductor is detached from a support occurs under the fuel cell operation environment, or operation time is delayed or physical shape stability is deteriorated under the low humidity conditions, and when the wettability exceeds 15 cm, durability may be deteriorated or the ion conductor may be detached from the support due to accelerated swelling of the ion conductor under the fuel cell operation environment.

The nanoweb may have a contact angle of 90° or less, preferably 1 to 50°, more preferably 5 to 35°. The contact angle is obtained by charging distilled water in a syringe while maintaining 30° C. and RH of 40%, dropping a water drop with a diameter of 3 mm on the nanoweb, allowing the water drop to spread for 5 minutes and then measuring a contact angle formed between a separation membrane and the water drop. When the contact angle is less than 1°, wettability of the nanoweb is excellent, but it may be difficult to produce a nanoweb with high quality due to excessively high content of additive, and when the contact angle exceeds 90°, it may be difficult for the nanoweb to exhibit sufficient performance due to deteriorated wettability when used as a membrane for electrochemical devices.

When the nanofiber includes a hydrophobic polymer such as polyimide, it has advantages of superior heat resistance, chemical resistance and shape stability, but a great amount of an ion conducting polymer cannot be uniformly impregnated throughout the pores of the nanoweb due to lack of hydrophilicity, and ion conductivity may be deteriorated due to lack of formation of hydrophilic channel. Accordingly, hydrophilic treatment is required in order for the nanoweb including a hydrophobic polymer described above to satisfy the moisture content saturation time, moisture regain, wettability in accordance with wicking test or contact angle. Any hydrophilic treatment may be used without particular limitation in the present invention so long as it is a conventional method capable of improving hydrophilicity of nanowebs.

As an example of the hydrophilic treatment of the nanoweb, the nanoweb may include a hydrophilic additive. That is, the nanofiber may include the hydrophilic additive therein, the hydrophilic additive may be impregnated in pores of the nanoweb, or the hydrophilic additive may be coated on one or both surfaces of the nanoweb.

Specifically, when the nanofiber includes the hydrophilic additive, the nanofiber may include 0.1 to 20 parts by weight, preferably 0.5 to 20 parts by weight, more preferably 1 to 2 parts by weight of the hydrophilic additive with respect to 100 parts by weight of the nanofiber polymer.

When the content of the hydrophilic additive is less than 0.1 parts by weight, with respect to 100 parts by weight of the nanofiber polymer, wettability and thus performance of electrochemical devices are deteriorated due to lack of hydrophilicity and, when the content exceeds 20 parts by weight, instability of nanofiber jet during the spinning process is increased, fibers are non-uniformly collected, and a problem may occur when applied to a separation membrane for electrochemical devices.

When the hydrophilic additive is impregnated in pores of the nanoweb, or the hydrophilic additive is coated on one or both surfaces of the nanoweb, the nanoweb may include 0.1 to 20 parts by weight, preferably 3 to 20 parts by weight, more preferably 5 to 20 parts by weight of the hydrophilic additive with respect to 100 parts by weight of the nanoweb.

When the content of the hydrophilic additive is less than 0.1 parts by weight, with respect to 100 parts by weight of the nanoweb, wettability and thus performance of electrochemical devices are deteriorated due to lack of hydrophilicity, and when the content exceeds 20 parts by weight, instability of nanofiber jet during the spinning process is increased, fibers are non-uniformly collected, and a problem may occur when applied to a separation membrane for electrochemical devices.

As the nanoweb includes the hydrophilic additive, it has superior wettability and excellent wettability to the electrolyte when used for a separation membrane for electrochemical devices, thereby improving efficiency of cells. In addition, the porous support has excellent durability, heat resistance and chemical resistance, thus maintaining performance of electrochemical devices even under harsh operation conditions.

The hydrophilic additive may be an inorganic or organic hydrophilic additive. Any inorganic hydrophilic additive may be used without particular limitation so long as it does not cause oxidation and/or reduction reactions, that is, electrochemical reactions with an anode or cathode collector within an operation voltage range (for example, 0 to 5V based on Li/Li⁺ in the case of a lithium secondary battery) of an electrochemical device, does not impair conductivity and endures the process of producing nanofibers including the same.

For example, the inorganic hydrophilic additive may be selected from the group consisting of TiO₂ anatase, TiO₂ rutile, TiO₂ brookite, tin dioxide (SnO), zirconium dioxide (ZrO₂) aluminium oxide (Al₂O₃), oxidized single-walled carbon nanotubes, oxidized multiwalled carbon nanotubes, graphite oxide, graphene oxide and a combination thereof and is preferably TiO₂.

In addition, any organic hydrophilic additive may be used without particular limitation so long as it does not cause oxidation and/or reduction reactions, that is, electrochemical reactions with an anode or cathode collector within an operation voltage range (for example, 0 to 5V based on Li/Li⁺ in the case of a lithium secondary battery) of an electrochemical device, does not impair conductivity and endures the process of producing nanofibers including the same.

For example, the organic hydrophilic additive is any one selected from the group consisting of polyhydroxyethylmethacrylate, polyvinylacetate, polyurethane, polydimethylsiloxane, polyimide, polyamide, polyethyleneterephthalate, polymethylmethacrylate, epoxy and a combination thereof.

The hydrophilic additive may be a nanohydrophilic additive and has thus a mean diameter of 0.005 to 1 μm, preferably 0.005 to 0.8 μm, more preferably 0.005 to 0.5 μm. When the mean diameter of the nano hydrophilic additive is less than 0.005 μm, nano hydrophilic particles are aggregated, thus inhibiting hydrophilic effect or making it difficult to handle, and when the mean diameter of the nano hydrophilic additive exceeds 1 μm, physical tensile strength of the support is deteriorated and elongation at break is decreased.

As described above, when the nanofiber includes polyimide as a hydrophobic polymer, in order to satisfy the moisture content saturation time, moisture regain, wettability in accordance with wicking test or contact angle, the main chain of the polyimide may include any one hydrophilic substituent selected from the group consisting of an amine group, a carboxyl group, a hydroxyl group and a combination thereof.

That is, the polyimide may be prepared by preparing polyamic acid (PAA) and then conducting imidization during a subsequent curing process. The polyamic acid may be prepared by an ordinary preparation method and specifically, by mixing diamine with a solvent, adding dianhydride thereto and conducting polymerization, and the diamine is aromatic diamine and the dianhydride is preferably fully aromatic polyimide prepared from aromatic dianhydride.

In this case, in order to incorporate any one substituent selected from the group consisting of an amine group, a carboxyl group, a hydroxyl group and a combination thereof into the main chain of the polyimide, the polyimide or the polyamic acid is prepared and the main chain of the polyimide or the polyamic acid is then substituted by the hydrophilic substituent, or the polyimide is prepared from the diamine and/or the dianhydride including the hydrophilic substituent, or a comonomer having a hydroxyl group is incorporated during polymerization, rather than the diamine and the dianhydride. The comonomer having a hydroxyl group may be any one selected from the group consisting of dianiline having a hydroxyl group, diphenyl urea having a hydroxyl group, diamine having a hydroxyl group and a combination thereof.

When the main chain of polyimide includes the hydrophilic substituent, the hydrophilic substituent may be present in an amount of 0.01 to 0.1 mol %, preferably 0.01 to 0.08 mol %, more preferably 0.02 to 0.08 mol % with respect to the total weight of the polyimide. When the content of the hydrophilic substituent is less than 0.01 mol %, hydrophilicity may be insufficient due to reduction of the hydrophilic group in the polyimide main chain, and when the content exceeds 0.1 mol %, side-reactions may occur and physical strength and elongation may be deteriorated.

As described above, when the nanofiber includes a hydrophobic polymer such as polyimide, one or two surfaces of the nanoweb may be subjected to plasma treatment in order to satisfy the moisture content saturation time, moisture regain, wettability in accordance with wicking test or contact angle. When the nanoweb is subjected to plasma treatment, the nanoweb surface can be substituted by any one hydrophilic functional group selected from the group consisting of a carboxyl group, a hydroxyl group, an amine group and a combination thereof.

Specifically, the plasma treatment may be carried out by treating one or two surfaces of the nanoweb with a gas for imparting a hydrophilic group using low-temperature plasma or radio frequency (RF) plasma. The gas for imparting a hydrophilic group may be any one selected from the group consisting of ammonia gas, argon gas, oxygen gas and a combination thereof, a flow rate of the gas for imparting a hydrophilic group may be 10 to 200 sccm, a power of the plasma is 50 to 200 W, and plasma treatment time may be 10 sec to 5 min.

In addition, as described above, when the nanofiber includes a hydrophobic polymer such as polyimide, an inorganic substance may be deposited on one or two surfaces of the nanoweb in order to satisfy the moisture content saturation time, moisture regain, wettability in accordance with wicking test or contact angle. The inorganic substance may be any one selected from the group consisting of TiO₂ anatase, TiO₂ rutile, TiO₂ brookite, tin dioxide (SnO), zirconium dioxide (ZrO₂), aluminium oxide (Al₂O₃), oxidized single-walled carbon nanotubes, oxidized multiwalled carbon nanotubes, graphite oxide, graphene oxide and a combination thereof.

The porous support has superior gas permeability and water permeability as well as excellent heat resistance and chemical resistance, thus being useful for filter materials for gas or liquid filters, filter materials for dustproof masks, materials for filters such as vents for cars, vents for cellular phones and vents for printers, materials for high-quality clothing such as moisture-permeable waterproof fabrics, polymer electrolytes for fuel cells, secondary batteries, electrochemical materials such as separation membranes for electrolysis devices or capacitors, and medical materials such as dressings for wound treatment, supports for artificial vessels, bandages, and masks for cosmetics which require heat resistance and chemical resistance.

A method of manufacturing a porous support according to an embodiment of the present invention includes electrospinning an electrospinning solution to form a nanoweb in which nanofibers are integrated in the form of a non-woven fabric including a plurality of pores.

For example, when the nanofibers include polyimide as a hydrophobic polymer, the method of manufacturing a porous support includes adding diamine and dianhydride to a solvent to prepare an electrospinning solution, electrospinning the prepared electrospinning solution to produce a polyamic acid nanoweb integrated in the form of a non-woven fabric including a plurality of pores, and imidizing the polyamic acid nanoweb to produce a polyimide nanoweb.

Hereinafter, the respective steps will be described. The electrospinning solution is a solution which contains monomers for forming the nanofibers, the monomers for forming the nanofibers are preferably a hydrocarbon-based polymer which exhibits superior chemical resistance and hydrophobicity and is thus free from shape deformation by moisture under high humidity environments.

Specifically, the hydrocarbon-based polymer may be selected from the group consisting of nylon, polyimide, polyaramide, polyether imide, polyacrylonitrile, polyaniline, polyethylene oxide, polyethylene naphthalate, polybutylene terephthalate, styrene butadiene rubber, polystyrene, polyvinyl chloride, polyvinyl alcohol, polyvinylidene fluoride, polyvinyl butylene, polyurethane, polybenzoxazole, polybenzimidazole, polyamide-imide, polyethylene terephthalate, polyethylene, polypropylene, a copolymer thereof and a mixture thereof and is preferably polyimide which exhibits relatively better heat resistance, chemical resistance and shape stability. Hereinafter, an example in which a nano-fiber includes polyimide as a hydrophobic polymer will be described in detail.

Any monomer for forming the nanofibers may be used without particular limitation so long as it is capable of forming the hydrocarbon-based polymer. For example, the nanoweb including polyimide is prepared by producing a polyamic acid nanoweb using polyamic acid (PAA) which is a polyimide precursor which is readily dissolved in an organic solvent and conducting imidization during a subsequent curing process.

The polyamic acid nanoweb may be produced by an ordinary production method, specifically, by mixing diamine with a solvent, adding dianhydride thereto and then electrospinning the resulting mixture.

The dianhydride may be selected from the group consisting of pyromellitic dianhydride (PMDA), 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA), 4,4′-oxydiphthalic anhydride (ODPA), 3,4,3′,4′-biphenyltetracarboxylic dianhydride (BPDA), and bis(3,4-dicarboxyphenyl)dimethylsilane dianhydride (SiDA) and mixtures thereof. In addition, the diamine may be selected from the group consisting of 4,4′-oxydianiline (ODA), 1,3-bis(4-aminophenoxy)benzene (RODA), p-phenylene diamine (p-PDA), o-phenylene diamine (o-PDA) and mixtures thereof. The solvent used for dissolving the poly(amic acid) may be selected from the group consisting of m-cresol, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc), dimethylsulfoxide (DMSO), acetone, diethyl acetate, tetrahydrofuran (THF), chloroform, butyrolactone and mixtures thereof.

The monomers for forming nanofibers is preferably present in an amount of 5 to 20% by weight with respect to the total weight of the spinning solution. When the content of the polymer is less than 5% by weight, because spinning cannot smoothly proceed, fibers cannot be formed or fibers with a uniform diameter cannot be produced and, on the other hand, when the content of the monomers exceeds 20% by weight, spinning cannot be conducted or processability may be deteriorated due to significantly increased ejection pressure.

In step 2, the electrospinning solution is spun to produce a nanoweb precursor, that is, a polyamic acid nanoweb. There is no particular limitation as to spinning in the present invention, and the spinning is electrospinning, electro-blown spinning, centrifugal spinning or melt blowing or the like, preferably, electrospinning.

Hereinafter, an example of using electrospinning will be described in detail.

FIG. 1 is a schematic view illustrating a nozzle-type electrospinning device. Referring to FIG. 1, in accordance with electrospinning, a predetermined amount of the precursor solution is supplied from a solution tank 1 storing the nanofiber precursor solution to a nozzle 3 using a volumetric pump 2 and the nanofiber precursor solution is ejected through the nozzle 3 to form nanofiber precursors. At this time, the nanofiber precursors are scattered and, at the same time, coagulated. The coagulated nanofiber precursors are collected on the collector 4 to produce a precursor nanofiber of the porous support.

In this case, the electrospinning may be carried out under the conditions that a positive charge density near the nozzle is increased and a negative charge density near the collector is increased. As a result, when polymer droplets are spun and scattered, they repel one another, so that they can be advantageously collected as nanofibers. Near the nozzle or near the collector may mean an area which is within 10 cm from the surface of the nozzle or the collector, but the present invention is not particularly limited thereto.

Specifically, the positive charge density near the nozzle can be controlled by installing a high-voltage generator (not shown) for supplying a positive charge near the nozzle and the negative charge density near the collector can be controlled by installing a high-voltage generator (not shown) for supplying a negative charge near the collector.

A level of increasing the positive charge density near the nozzle can be controlled by supplying a positive charge of +10 to +100 kV near the nozzle, and a level of increasing the negative charge density near the collector can be controlled by supplying a negative charge of 0 to −100 kV near the collector. When the amount of the supplied positive charge is less than +10 kV, spinning capability may not be sufficient, when the amount exceeds +100 kV, electrical insulation may be removed and, when the amount of the supplied negative charge is less than zero, potential difference may not be sufficient and, when the amount of supplied negative charge exceeds −100 kV, insulation may be removed.

In this case, an intensity of electric field between the nozzle 3 and the collector 4 applied by a high-voltage generator 6 and a voltage transfer road 5 is preferably 850 to 3,500 V/cm. When the intensity of the electric field is less than 850 V/cm, uniform thickness of nanofibers cannot be produced because the precursor solution is not continuously ejected, and production of the nanoweb may be difficult because the nanofibers formed after spinning cannot be smoothly collected on the collector and, when the intensity of electric field exceeds 3,500 V/cm, the nanofibers are not mounted at a desired position on collector 4, thus making acquisition of a nanoweb having a normal shape impossible.

Nanofiber precursors having a uniform fiber diameter, preferably a mean diameter of 0.01 to 5 μm are produced by the spinning process, and the nanofiber precursors are arranged in a predetermined direction or randomly to form a non-woven fabric.

In step 3, the nanofiber precursor of the nanoweb precursor is cured.

In order to convert the nanofiber precursor into the nanofiber, a curing process which is an additional process performed on the nanofiber precursor is conducted. For example, when the nanofiber precursor produced by electrospinning includes polyamic acid, the nanofiber precursor is converted into polyimide by imidization during the curing process.

Accordingly, preferably, the temperature of the curing process is suitably controlled in consideration of conversion ratio of the nanofiber precursor. Specifically, the curing process may be conducted at 80 to 650° C. When the temperature during curing is lower than 80° C., conversion ratio is decreased and, as a result, the heat resistance and chemical resistance of the nanoweb may be deteriorated and, when the curing temperature exceeds 650° C., physical properties of the nanoweb may be deteriorated by degradation of the nanofibers.

Meanwhile, as described above, when the nanofiber includes a hydrophobic polymer such as polyimide, the nanoweb may include a hydrophilic additive in order to satisfy the moisture content saturation time, moisture regain, wettability in accordance with wicking test or contact angle. That is, the nanofiber may include the hydrophilic additive therein, the hydrophilic additive may be impregnated in pores of the nanoweb, or the hydrophilic additive may be coated on one or both surfaces of the nanoweb.

Specifically, when the nanofiber includes the hydrophilic additive, the hydrophilic additive may be further added to the electrospinning solution and electrospinning may be then conducted. In this case, the hydrophilic additive may include 0.1 to 20 parts by weight, preferably 3 to 20 parts by weight, more preferably 5 to 20 parts by weight of the hydrophilic additive, with respect to 100 parts by weight of the monomer for producing the nanofiber.

When the content of hydrophilic additive is less than 0.1 parts by weight, with respect to 100 parts by weight of the monomer for producing the nanofiber, wettability and thus performance of electrochemical devices are deteriorated due to lack of hydrophilicity and, when the content exceeds 20 parts by weight, instability of nanofiber jet during the spinning process is increased, fibers are non-uniformly collected, and a problem may occur when applied to a separation membrane for electrochemical devices.

In addition, spinning of the electrospinning solution can be carried out under general spinning conditions, but upon spinning of a precursor solution containing the hydrophilic additive, nanofibers cannot be uniformly collected on the collector due to increased instability of spinning jet and nanowebs having high quality cannot be produced. Accordingly, upon spinning of the precursor solution including the hydrophilic additive, a cation blower is installed near a spinning area to improve a cation density, and a base material for the collector surface is exposed to an anion blower to improve an anion density of a collector material. If not, stable spinning jet cannot be obtained and it may be difficult to produce a uniform and high-quality support.

When the hydrophilic additive is impregnated in pores of the nanoweb, or the hydrophilic additive is coated on one or both surfaces of the nanoweb, the porous support can be produced by impregnating the nanoweb, through immersion, in a hydrophilic additive solution prepared by adding the hydrophilic additive to a solvent, or coating the hydrophilic additive solution on the surface of the nanoweb.

The impregnation of the nanoweb in the hydrophilic additive solution may be carried out by immersing the nanoweb in the hydrophilic additive solution at room temperature (20° C.) for 5 to 30 minutes, then drying the same at 50 to 100° C. in an air oven for 3 hours or longer and repeating the immersing and drying operations two or five times.

In addition, the coating of the hydrophilic additive solution on the surface of the nanoweb may be carried out using a variety of methods well-known in the art such as a laminating, spraying, screen printing or doctor blade process.

The hydrophilic additive solution may be preparing by adding an inorganic hydrophilic additive selected from the group consisting of TiO₂ anatase, TiO₂ rutile, TiO₂ brookite, tin dioxide (SnO), zirconium dioxide (ZrO₂), aluminium oxide (Al₂O₃), oxidized single-walled carbon nanotubes, oxidized multiwalled carbon nanotubes, graphite oxide, graphene oxide and a combination thereof, or any one organic hydrophilic additive selected from the group consisting of polyhydroxyethylmethacrylate, polyvinylacetate, polyurethane, polydimethylsiloxane, polyimide, polyamide, polyethyleneterephthalate, polymethylmethacrylate, epoxy and a combination thereof to any one solvent selected from the group consisting of N-methyl-2-pyrrolidine (NMP), dimethylformamide (DMF), dimethylacetamide (DMA), dimethylsulfoxide (DMSO) and a combination thereof, followed by mixing.

As described above, when the produced nanoweb includes polyimide as a hydrophobic polymer, the main chain of polyimide may include any one hydrophilic substituent selected from the group consisting of an amine group, a carboxyl group, a hydroxyl group and a combination thereof in order to satisfy the moisture content saturation time, moisture regain, wettability in accordance with wicking test or contact angle.

That is, in order to produce a nanoweb including polyimide including the hydrophilic substituent in a main chain, the porous support can be produced by preparing the polyimide or the polyamic acid and then substituting the main chain of the polyimide or the polyamic acid by the hydrophilic substituent, or preparing the polyimide from the diamine and/or the dianhydride including the hydrophilic substituent, or incorporating the comonomer having a hydroxyl group during polymerization, in addition to the diamine and the dianhydride.

The substitution of the main chain of the polyimide or the polyamic acid by the hydrophilic substituent may be carried out by substituting a part of the main chain by a carboxyl group and an amine group by treatment with an alkaline aqueous solution such as KOH or NaOH.

In addition, any one may be used as the comonomer having a hydroxyl group so long as it includes the hydrophilic substituent and can be polymerized with the diamine and/or the dianhydride. For example, the comonomer having a hydroxyl group may be any one selected from the group consisting of dianiline having a hydroxyl group, diphenyl urea having a hydroxyl group, diamine having a hydroxyl group and a combination thereof.

As described above, when the nanofiber includes a hydrophobic polymer such as polyimide, one or two surfaces of the nanoweb may be subjected to plasma treatment in order to satisfy the moisture content saturation time, moisture regain, wettability in accordance with wicking test or contact angle. When the nanoweb is subjected to plasma treatment, the nanoweb surface can be substituted by any one hydrophilic functional group selected from the group consisting of a carboxyl group, a hydroxyl group, an amine group and a combination thereof.

Specifically, the plasma treatment may be carried out by treating one or two surfaces of the nanoweb with a gas for imparting a hydrophilic group using low-temperature plasma or radio frequency (RF) plasma. The gas for imparting a hydrophilic group may be any one selected from the group consisting of ammonia gas, argon gas, oxygen gas and a combination thereof, a flow rate of the gas for imparting a hydrophilic group may be 10 to 200 sccm, a power of the plasma is 50 to 200 W, and plasma treatment time may be 10 sec to 5 min.

As described above, when the nanofiber includes a hydrophobic polymer such as polyimide, an inorganic substance may be deposited on one or two surfaces of the nanoweb in order to satisfy the moisture content saturation time, moisture regain, wettability in accordance with wicking test or contact angle.

The deposited inorganic substance layer can be formed by depositing any one precursor selected from the group consisting of TiO₂ anatase, TiO₂ rutile, TiO₂ brookite, tin dioxide (SnO), zirconium dioxide (ZrO₂), aluminium oxide (Al₂O₃), oxidized single-walled carbon nanotubes, oxidized multiwalled carbon nanotubes, graphite oxide, graphene oxide and a combination thereof, by chemical vapor deposition (CVD) or physical vapor deposition (PVD) including sputtering. The deposition may be carried out by disposing a target for imparting a hydrophilic group on the surface using an RF sputter or depositor and treating at a temperature of 50 to 300° C. for 1 to 60 minutes.

In accordance with another embodiment of the present invention, provided is a reinforced membrane which includes the porous support and an ion exchange polymer filling pores of the porous support.

A method of filling an ion exchange polymer in pores of the porous support is for example impregnation. The impregnation may be carried out by dipping the porous support in a solution containing an ion exchange polymer. In addition, the ion exchange polymer may be formed by dipping an associated monomer or low molecular weight oligomer in the porous support and polymerizing in-situ the same in the porous support.

The impregnation temperature and time may be affected by various parameters. For example, the impregnation temperature and time may be affected by the thickness of the nanoweb, concentration of the ion exchange polymer, the type of solvent, concentration of ion exchange polymer to be impregnated in the porous support and the like. The impregnation process may be carried out at a temperature of not less than a freezing point of the solvent and not higher than 100° C., more typically at room temperature (20° C.) to a temperature of 70° C. or lower. The temperature cannot be a melting point or higher of the nanofibers.

The ion exchange polymer may be a cation exchange polymer having a cation exchange group such as a proton, or an anion exchange polymer having an anion exchange group such as a hydroxyl, carbonate or bicarbonate ion.

The cation exchange group may be any one selected from the group consisting of a sulfonic acid group, a carboxyl group, a boronic acid group, a phosphoric acid group, an imide group, a sulfonimide group, a sulfonamide group and a combination thereof and is generally a sulfonic acid group or a carboxyl group.

The cation exchange polymer includes the cation exchange group and examples thereof include fluoro-based polymers containing fluorine in a main chain; hydrocarbon-based polymers such as benzimidazole, polyamide, polyamideimide, polyimide, polyacetal, polyethylene, polypropylene, acrylic resins, polyester, polysulfone, polyether, polyetherimide, polyester, polyethersulfone, polyetherimide, polycarbonate, polystyrene, polyphenylenesulfide, polyetheretherketone, polyetherketone, polyarylethersulfone, polyphosphazene or polyphenylquinoxaline; partially fluorinated polymers such as polystyrene-graft-ethylenetetrafluoroethylene copolymers or polystyrene-graft-polytetrafluoroethylene copolymers; and sulfone imide.

More specifically, when the cation exchange polymer is a hydrogen ion cation exchange polymer, the polymers may include, in a side chain, a cation exchange group selected from the group consisting of a sulfonic acid group, a carboxylic acid group, a phosphoric acid group, a phosphonic acid group and a derivative thereof, and examples thereof include, but are not limited to, a fluoro-based polymer including poly(perfluorosulfonic acid), poly(perfluorocarboxylic acid) including a sulfonic acid group, a copolymer of tetrafluoroethylene including a sulfonic acid group with fluorovinylether, defluorinated sulfide polyetherketone or a mixture thereof; and a hydrocarbon-based polymer including sulfonated polyimide (S-PI), sulfonated polyarylethersulfone (S-PAES), sulfonated polyetheretherketone (SPEEK), sulfonated polybenzimidazole (SPBI), sulfonated polysulfone (S-PSU), sulfonated polystyrene (S-PS), sulfonated polyphosphazene and a mixture thereof.

The anion exchange polymer is a polymer which is capable of transferring an anion such as a hydroxyl, carbonate or bicarbonate ion, commercially available anion exchange polymers are hydroxides or halides (generally, chloride), and the anion exchange polymer may be used for industrial water purifications, metal separation or catalyst process and the like.

The anion exchange polymer is generally a metal hydroxide-doped polymer and specifically, is metal hydroxide-doped poly(ethersulfone), polystyrene, vinyl-based polymers, poly(vinyl chloride), poly(vinylidene fluoride), poly(tetrafluoroethylene), poly(benzimidazole) or poly(ethyleneglycol) or the like.

The ion exchange polymer may be present in an amount of 50 to 99% by weight with respect to the total weight of the reinforced membrane. When the content of the ion exchange polymer is less than 50% by weight, ion conductivity of the reinforced membrane may be deteriorated and, when the content of the ion exchange polymer exceeds 99% by weight, the mechanical strength and dimensional stability of the reinforced membrane may be deteriorated.

When the ion exchange polymers are filled in pores of the porous support, a coating layer may be formed on one or two surfaces of the porous support during the production process. The thickness of the coating layer of the ion exchange polymer is preferably controlled to 30 μm or less. When the coating layer of the ion exchange polymer is formed to a thickness of higher than 30 μm on the surface of the porous support, the mechanical strength of the reinforced membrane may be deteriorated, the total thickness of the reinforced membrane is increased and resistance loss is thus increased.

The reinforced membrane has a structure in which the ion exchange polymer is filled in pores of the porous support, thus exhibiting superior mechanical strength of 40 MPa or more. As such, as mechanical strength increases, the total thickness of the reinforced membrane can be reduced to 80 μm or less and, as a result, material costs are reduced, ion conduction speed is increased and resistance loss is reduced.

In addition, the reinforced membrane includes a porous support having superior durability and superior binding capability between nanofibers and the ion exchange polymer constituting the porous support, thereby preventing three-dimensional expansion of the reinforced membrane resulting from moisture and reducing length and thickness increase fractions. Specifically, the reinforced membrane has superior dimensional stability of 5% or less when swollen in water. The dimensional stability is a physical property which is evaluated in accordance with the following Equation 4 from variation in length before and after swelling of the reinforced membrane.

Dimensional stability=[(length after swelling−length before swelling)/length before swelling]×100  [Equation 4]

The reinforced membrane has superior dimensional stability and ion conductivity, thus being preferably useful for polymer electrolyte membranes for fuel cells or membranes for reverse osmosis filters.

MODE FOR INVENTION

Hereinafter, embodiments according to the present invention will be described in detail to such an extent that a person having ordinary knowledge in the art field to which the invention pertains can easily carry out the invention. However, the present invention can be realized in various forms and is not limited to embodiments stated herein.

PRODUCTION EXAMPLE Production of Porous Support Example 1-1

100 parts by weight of PMDA, ODA and PDA monomers, and parts by weight of nano TiO₂ anatase as a hydrophilic additive were dissolved in a dimethylformamide solvent to prepare 5 L of a spinning solution having a solid content of 12.5% by weight and a viscosity of 620 poise. The prepared spinning solution was transferred to a solution tank, fed by a volumetric gear pump to a spinning chamber having 26 nozzles and to which a high voltage of 49 kV was applied, and then spun to produce a polyamic acid nanoweb. At this time, the amount of supplied solution was 1.0 ml/min. In addition, a cation blower and an anion blower were installed on a spinning chamber and a collector material, respectively, to improve cation density based on the spinning environment and anion density based on the collector material.

Subsequently, the polyamic acid nanoweb was transferred by a roll-to-roll method and heat-cured in a continuous curing furnace at a temperature of 420° C. for 10 minutes to produce a porous support including a polyimide nanoweb.

Example 1-2

A porous support was produced in the same manner as in Example 1-1 except that 0.1 parts by weight of nano TiO₂ anatase was used as the hydrophilic additive.

Example 1-3

A porous support was produced in the same manner as in Example 1-1 except that 20 parts by weight of nano TiO₂ anatase was used as the hydrophilic additive.

Comparative Example 1-1

100 parts by weight of PMDA, ODA and PDA monomers, and 5 parts by weight of nano TiO₂ anatase as a hydrophilic additive were dissolved in a dimethylformamide solution to prepare 5 L of a spinning solution having a solid content of 12.5% by weight and a viscosity of 620 poise. The prepared spinning solution was transferred to a solution tank, fed by a volumetric gear pump to a spinning chamber having 26 nozzles and to which a high voltage of 49 kV was applied, and then spun to produce a polyamic acid nanoweb. At this time, the amount of supplied solution was 1.0 ml/min.

Subsequently, the polyamic acid nanoweb was heat-cured in a continuous curing furnace at a temperature of 420° C. for minutes to produce a porous support including a polyimide nanoweb.

Test Example 1 Measurement of Properties of Porous Support

The moisture content saturation time, moisture regain, wettability and contact angle of the porous supports produced in Examples and Comparative Examples were measured and results are summarized in the following Table 1.

TABLE 1 Comparative Example Example Example Example 1-1 1-2 1-3 1-1 Moisture 200 600 60 3600 content saturation time¹⁾ Moisture 3.5 3.0 5.0 2.5 regain²⁾ Wettability³⁾ 3 2 7 0 contact 45 80 25 113 angle⁴⁾ ¹⁾Moisture content saturation time (sec): the moisture content saturation time was obtained from a time for which a sample was completely wet with water which was dropped from a height of 25 mm in accordance with KS K ISO 9073-6, textile-non-woven test method-Part VI: liquid absorption measurement method of absorption measurement standard. ²⁾Moisture regain (% by weight): was obtained by measuring, in accordance with KS K 0221, method of absorbing moisture of textile: oven balance method, the weight of a sample (O) after the sample reaches moisture equilibrium for 24 hours under laboratory standard conditions (KS K 0901), measuring the weight of the dried sample (D) at 105 to 110 C.° for 90 minutes and then calculating a weight variation ³⁾Wettability (cm): was measured by immersing a sample for 30 minutes and then measuring a wicking maximum distance in accordance with USA AATCC Test Method 197-2011, Option B, Measure distance at a given time in Vertical Wicking of Textiles. ⁴⁾Contact angle (°): was measured by charging distilled water in a syringe while maintaining 30° C. and RH of 40%, dropping a water drop with a diameter of 3 mm on the nanoweb, allowing the water drop to spread for 5 minutes and then measuring a contact angle formed between a separation membrane and the water drop.

As can be seen from Table 1, the porous membrane produced in Example has superior hydrophilicity as compared to the porous membrane produced in Comparative Example.

Production Example 2 Production of Porous Support Example 2-1

PMDA, ODA and PDA monomers were dissolved in a dimethylformamide solvent to prepare 5 L of a spinning solution having a solid content of 12.5% by weight and a viscosity of 620 poise. The prepared spinning solution was transferred to a solution tank, fed by a volumetric gear pump to a spinning chamber having 26 nozzles and to which a high voltage of 49 kV was applied, and then spun to produce a polyamic acid nanoweb. At this time, the amount of supplied solution was 1.0 ml/min.

Subsequently, the polyamic acid nanoweb was heat-cured in a continuous curing furnace maintained at a temperature of 420° C. for 10 minutes to produce a polyimide nanoweb.

Meanwhile, nano TiO₂ anatase as a hydrophilic additive was dissolved in a dimethylformamide solvent and stirred to prepare a hydrophilic additive solution. The produced nanoweb was immersed in the prepared hydrophilic additive solution at room temperature (20° C.) for 5 to 30 minutes, was dried at 50 to 100° C. in an air oven for 3 hours or longer and the immersing and drying operations were repeated two or five times to impregnate the hydrophilic additive in the nanoweb.

Example 2-2

A porous support was produced in the same manner as in Example 2-1 except that coating the hydrophilic additive solution on two surfaces of the nanoweb by spraying and then drying the same were repeated.

Example 2-3

PMDA, ODA and PDA monomers were dissolved in a dimethylformamide solvent to prepare 5 L of a spinning solution having a solid content of 12.5% by weight and a viscosity of 620 poise. The prepared spinning solution was transferred to a solution tank, fed by a volumetric gear pump to a spinning chamber having 26 nozzles and to which a high voltage of 49 kV was applied, and then spun to produce a polyamic acid nanoweb. At this time, the amount of supplied solution was 1.0 ml/min.

Subsequently, the polyamic acid nanoweb was heat-cured in a continuous curing furnace maintained at a temperature of 420° C. for 10 minutes to produce a polyimide nanoweb. The two surfaces of the produced polyimide nanoweb were fed into a plasma treatment chamber, oxygen gas was fed thereto at a flow rate of 150 sccm using low-temperature plasma and plasma treatment was conducted at 20 W for 5 minutes.

Example 2-4

PMDA, ODA and PDA monomers were dissolved in a dimethylformamide solvent to prepare 5 L of a spinning solution having a solid content of 12.5% by weight of and a viscosity of 620 poise. The prepared spinning solution was transferred to a solution tank, fed by a volumetric gear pump to a spinning chamber having 26 nozzles and to which a high voltage of 49 kV was applied, and then spun to produce a polyamic acid nanoweb. At this time, the amount of supplied solution was 1.0 ml/min.

Subsequently, the polyamic acid nanoweb was heat-cured in a continuous curing furnace maintained at a temperature of 420° C. for 6 minutes to produce a polyimide nanoweb. The two surfaces of the produced polyimide nanoweb was sputtered at a constant deposition power of 150 W and a constant sample temperature of 200° C. using an RF sputter for 10 minutes to form a TiO₂ inorganic substance layer.

Test Example 2 Measurement of Properties of Porous Support

The moisture content saturation time, moisture regain, wettability and contact angle of the porous supports produced in Examples and Comparative Examples were measured and results are summarized in the following Table 2.

TABLE 2 Exam- Comparative Example Example Example ple Example 2-1 2-2 2-3 2-4 1-1 Moisture 250 280 10 300 3600 content saturation time¹⁾ Moisture 3.3 3.6 5.0 3.8 2.5 regain²⁾ Wettability³⁾ 4 4.5 15 6 0 Contact 38 43 10 30 113 angle⁴⁾ ¹⁾Moisture content saturation time (sec): the moisture content saturation time was obtained from a time for which a sample was completely wet with water which was dropped from a height of 25 mm in accordance with KS K ISO 9073-6, textile-non-woven test method-Part VI: liquid absorption measurement method of absorption measurement standard. ²⁾Moisture regain (% by weight): was obtained by measuring, in accordance with KS K 0221, method of absorbing moisture of textile: oven balance method, the weight of a sample (O) after the sample reaches moisture equilibrium for 24 hours under laboratory standard conditions (KS K 0901), measuring the weight of the dried sample (D) at 105 to 110° C. for 90 minutes and then calculating a weight variation ³⁾Wettability (cm): was measured by immersing a sample for 30 minutes and then measuring a wicking maximum distance in accordance with USA AATCC Test Method 197-2011, Option B, Measure distance at a given time in Vertical Wicking of Textiles. ⁴⁾Contact angle (°): was measured by charging distilled water in a syringe while maintaining 30° C. and RH of 40%, dropping a water drop with a diameter of 3 mm on the nanoweb, allowing the water drop to spread for 5 minutes and then measuring a contact angle formed between a separation membrane and the water drop.

As can be seen from Table 2, the porous membrane produced in Example has superior hydrophilicity as compared to the porous membrane produced in Comparative Example.

Production Example 3 Production of Porous Support Example 3-1

PMDA, ODA and PDA, and a hydroxyl group-containing diphenyl urea monomer were dissolved at a ratio of 50:45:5 in a dimethylformamide solvent to prepare 5 L of a spinning solution having a solid content of 12.5% by weight and a viscosity of 620 poise.

The prepared spinning solution was transferred to a solution tank, fed by a volumetric gear pump to a spinning chamber having 26 nozzles and to which a high voltage of 49 kV was applied, and then spun to produce a polyamic acid nanoweb. At this time, the amount of supplied solution was 1.0 ml/min.

Subsequently, the polyamic acid nanoweb was heat-cured in a continuous curing furnace maintained at a temperature of 420° C. for 10 minutes to produce a polyimide nanoweb.

Example 3-2

PMDA, ODA and PDA monomers were dissolved in a dimethylformamide solvent to prepare 5 L of a spinning solution having a solid content of 12.5% by weight and a viscosity of 620 poise. The prepared spinning solution was transferred to a solution tank, fed by a volumetric gear pump to a spinning chamber having 26 nozzles and to which a high voltage of 49 kV was applied, and then spun to produce a polyamic acid nanoweb. At this time, the amount of supplied solution was 1.0 ml/min.

Subsequently, the polyamic acid nanoweb was heat-cured in a continuous curing furnace maintained at a temperature of 420° C. for 10 minutes to produce a polyimide nanoweb.

A 0.1N KOH solution was sprayed on the surface of the produced polyimide nanoweb using a spray for one second and then dried to substitute a main chain of the polyimide by a carboxyl group in 0.02 mol %.

Example 3-3

PMDA, ODA and PDA monomers were dissolved in a dimethylformamide solvent to prepare 5 L of a spinning solution having a solid content of 12.5% by weight and a viscosity of 620 poise. The prepared spinning solution was transferred to a solution tank, fed by a volumetric gear pump to a spinning chamber having 26 nozzles and to which a high voltage of 49 kV was applied, and then spun to produce a polyamic acid nanoweb. At this time, the amount of supplied solution was 1.0 ml/min.

Subsequently, the polyamic acid nanoweb was heat-cured in a continuous curing furnace maintained at a temperature of 420° C. for 10 minutes to produce a polyimide nanoweb.

A 0.1N KOH solution was sprayed on the surface of the produced polyimide nanoweb using a spray for one second and then dried to substitute a main chain of the polyimide by a carboxyl group in 0.01 mol %.

Test Example 3 Measurement of Properties of Porous Support

The moisture content saturation time, moisture regain, wettability and contact angle of the porous supports produced in Examples and Comparative Examples were measured and results are summarized in the following Table 3.

TABLE 3 Example Comparative Example 3-1 Example 3-2 3-3 Example 1-1 Moisture content 500 580 600 3600 saturation time¹⁾ Moisture regain²⁾ 3.1 3.3 3.2 2.5 Wettability³⁾ 2.5 2.9 2.8 0 Contact angle⁴⁾ 48 42 44 113 ¹⁾Moisture content saturation time (sec): the moisture content saturation time was obtained from a time for which a sample was completely wet with water which was dropped from a height of 25 mm in accordance with KS K ISO 9073-6, textile-non-woven test method-Part VI: liquid absorption measurement method of absorption measurement standard. ²⁾Moisture regain (% by weight): was obtained by measuring, in accordance with KS K 0221, method of absorbing moisture of textile: oven balance method, the weight of a sample (O) after the sample reaches moisture equilibrium for 24 hours under laboratory standard conditions (KS K 0901), measuring the weight of the dried sample (D) at 105 to 110° C. for 90 minutes and then calculating a weight variation ³⁾Wettability (cm): was measured by immersing a sample for 30 minutes and then measuring a wicking maximum distance in accordance with USA AATCC Test Method 197-2011, Option B, Measure distance at a given time in Vertical Wicking of Textiles. ⁴⁾Contact angle (°): was measured by charging distilled water in a syringe while maintaining 30° C. and RH of 40%, dropping a water drop with a diameter of 3 mm on the nanoweb, allowing the water drop to spread for 5 minutes and then measuring a contact angle formed between a separation membrane and the water drop.

As can be seen from Table 3, the porous membrane produced in Example has superior hydrophilicity as compared to the porous membrane produced in Comparative Example.

Production Example 4 Production of Reinforced Membrane

The porous supports produced in Production Examples 1 to 3 and 5% by weight of a Nafion solution were fed onto a petri dish such that 0.06 g of Nafion was impregnated per a unit area (cm²) of the web and was then dried at 60° C. in an oven for 4 hours or longer to produce a reinforced membrane.

Test Example 4

The reinforced membrane produced in Production Example 4 was immersed in a 1M sulfuric acid solution for 3 hours to sufficiently activate the hydrophilic group and the surface thereof was washed with ultrapure water to prepare a sample for measurement of conductivity, and ion conductivity was measured at a humidity of 90% and at 25° C. and 80° C. by a 4-electrode method.

In addition, the reinforced membrane produced in Production Example 4 was dried at 60° C. in an oven for 6 hours or longer and then stored in 80° C. hot water for 2 hours, the drying and storing operations were repeated 5 times, tensile strength of the reinforced membrane was measured with UTM-3365 equipment and whether or not detachment occurred was observed.

In addition, in order to evaluate shape stability of the reinforced membrane produced in Production Example 4, the reinforced membrane was dried in a hot air oven at a temperature of 50° C. for 6 hours and immersed in ultrapure water for 24 hours and dimensional variation of the reinforced membrane was measured.

Measurement results are shown in the following Table 4.

TABLE 4 Tensile strength Ionic (MPa) Dimensional conductivity After variation (Scm⁻¹) Before measurement 5 (%) 25° C. 80° C. evaluation times Detachment X Y Example 0.07 0.08 47 45 Detachment X 1 1 1-1 Example 0.06 0.07 40 33 Detachment X 0.5 0.5 1-2 Example 0.09 0.10 49 49 Detachment X 0.1 0.1 1-3 Example 0.07 0.08 46 43 Detachment X 0.5 0.5 2-1 Example 0.07 0.08 46 43 Detachment X 0.5 0.5 2-2 Example 0.08 0.09 40 35 Detachment X 1.0 1.0 2-3 Example 0.07 0.08 43 43 Detachment X 0.5 0.5 2-4 Example 0.07 0.08 41 40 Detachment X 0.5 0.5 3-1 Example 0.07 0.08 40 35 Detachment X 0.5 0.5 3-2 Example 0.07 0.08 40 35 Detachment X 0.5 0.5 3-3 Comparative 0.06 0.07 40 30 Detachment 1.0 1.0 Example 1-1 Comparative 0.09 0.10 36 26 Detachment 10.0 10.0 Example¹⁾ ¹⁾Control Example: polymer electrolyte membrane prepared by immersing the Nafion 117 membrane available from DuPont in ultrapure water for 3 hours such that water was sufficiently present in the membrane.

As can be seen from Table 4 above, the reinforced membranes of Examples and the reinforced membranes of Comparative Examples exhibited similar or identical hydrogen ionic conductivity at 25° C., as compared to the fluoro-based reinforced membrane of Control Example known to exhibit superior hydrogen ionic conductivity. However, at a high temperature of 80° C., the reinforced membrane of Example exhibited similar or identical hydrogen ion conductivity to the fluoro-based reinforced membrane of Comparative Example, whereas the reinforced membrane of Comparative Example exhibited significantly deteriorated ion conductivity, as compared to the fluoro-based reinforced membrane of Control Example.

In addition, the reinforced membrane of Example exhibited significantly improved shape stability, as compared to the fluoro-based reinforced membrane of Control Example known to exhibit superior hydrogen ion conductivity.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appropriate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

INDUSTRIAL APPLICABILITY

The porous support according to the present invention has wide surface area and excellent porosity, thus being useful for a variety of applications such as filters for water purification, filters for air purification, composites, membranes for cells and the like, in particular, being useful for reinforced composite membranes for fuel cells for cars. 

1. A porous support comprising a nanoweb in which nanofibers are integrated in the form of a non-woven fabric including a plurality of pores, wherein the nanoweb has a moisture content saturation time of 1 sec to 600 sec.
 2. The porous support according to claim 1, wherein the nanoweb has a moisture regain of 3.0% by weight or more.
 3. The porous support according to claim 1, wherein the nanoweb has wettability in accordance with wicking test, of 2 to 15 cm.
 4. The porous support according to claim 1, wherein the nanoweb has a contact angle of 90° or less.
 5. The porous support according to claim 1, wherein the nanofiber comprises 0.1 to 20 parts by weight of a polymer hydrophilic additive, with respect to 100 parts by weight of the nanofiber.
 6. The porous support according to claim 1, wherein a hydrophilic additive is impregnated in the pores of the nanoweb.
 7. The porous support according to claim 1, wherein a hydrophilic additive is coated on one or two surfaces of the nanoweb.
 8. The porous support according to claim 5, wherein the hydrophilic additive is selected from the group consisting of TiO₂ anatase, TiO₂ rutile, TiO₂ brookite, tin dioxide (SnO), zirconium dioxide (ZrO₂), aluminium oxide (Al₂O₃), oxidized single-walled carbon nanotubes, oxidized multiwalled carbon nanotubes, graphite oxide, graphene oxide and a combination thereof.
 9. The porous support according to claim 5, wherein the hydrophilic additive is selected from the group consisting of polyhydroxyethylmethacrylate, polyvinylacetate, polyurethane, polydimethylsiloxane, polyimide, polyamide, polyethyleneterephthalate, polymethylmethacrylate, epoxy and a combination thereof.
 10. The porous support according to claim 5, wherein the hydrophilic additive has a mean diameter of 0.005 to 1 μm.
 11. The porous support according to claim 1, wherein the nanofiber comprises a polyimide nanofiber.
 12. The porous support according to claim 11, wherein the polyimide has a main chain comprising a substituent selected from the group consisting of an amine group, a carboxyl group, a hydroxyl group and a combination thereof.
 13. The porous support according to claim 12, wherein the polyimide is prepared by polymerizing diamine, dianhydride and a comonomer containing a hydroxyl group to prepare polyamic acid and then imidizing the polyamic acid.
 14. The porous support according to claim 13, wherein the comonomer containing a hydroxyl group is selected from the group consisting of dianiline containing a hydroxyl group, diphenyl urea containing a hydroxyl group, diamine containing a hydroxyl group and a combination thereof.
 15. The porous support according to claim 1, wherein One or two surfaces of the nanoweb is plasma-treated.
 16. The porous support according to claim 1, wherein an inorganic substance is deposited on one or two surfaces of the nanoweb.
 17. The porous support according to claim 16, wherein the inorganic substance is selected from the group consisting of TiO₂ anatase, TiO₂ rutile, TiO₂ brookite, tin dioxide (SnO), zirconium dioxide (ZrO₂), aluminium oxide (Al₂O₃), oxidized single-walled carbon nanotubes, oxidized multiwalled carbon nanotubes, graphite oxide, graphene oxide and a combination thereof.
 18. A method of producing a porous support comprising: adding diamine and dianhydride to a solvent to prepare an electrospinning solution; electrospinning the prepared electrospinning solution to produce a polyamic acid nanoweb in which nanofibers are integrated in the form of a non-woven fabric including a plurality of pores; and imidizing the polyamic acid nanoweb to produce a polyimide nanoweb, wherein the polyimide nanoweb has a moisture content saturation time of 1 sec to 600 sec.
 19. The method according to claim 18, wherein a comonomer containing a hydroxyl group is further added to the electrospinning solution.
 20. The method according to claim 18, wherein the nanofibers present in the polyimide nanoweb have a main chain substituted by a substituent selected from the group consisting of an amine group, a carboxyl group, a hydroxyl group and a combination thereof.
 21. A method of producing a porous support comprising electrospinning an electrospinning solution to produce a nanoweb in which nanofibers are integrated in the form of a non-woven fabric including a plurality of pores, wherein the nanoweb has a moisture content saturation time of 1 sec to 600 sec.
 22. The method according to claim 21, wherein a hydrophilic additive is further added to the electrospinning solution.
 23. The method according to claim 21, further comprising: impregnating a hydrophilic additive in pores of the nanoweb.
 24. The method according to claim 21, further comprising: coating a hydrophilic additive on one or two surfaces of the nanoweb.
 25. The method according to claim 21, further comprising: plasma-treating one or two surfaces of the nanoweb.
 26. The method according to claim 25, wherein the plasma treatment is carried out by treating one or two surfaces of the nanoweb with gas for imparting a hydrophilic group using low-temperature plasma or radio frequency (RF) plasma.
 27. The method according to claim 21, further comprising: depositing an inorganic substance on one or two surfaces of the nanoweb.
 28. The method according to claim 27, wherein the deposition of the inorganic substance is carried out by sputtering.
 29. A reinforced membrane comprising: the porous support according to claim 1; and an ion exchange polymer filling pores of the porous support.
 30. The porous support according to claim 6, wherein the hydrophilic additive is selected from the group consisting of TiO₂ anatase, TiO₂ rutile, TiO₂ brookite, tin dioxide (SnO), zirconium dioxide (ZrO₂), aluminium oxide (Al₂O₃), oxidized single-walled carbon nanotubes, oxidized multiwalled carbon nanotubes, graphite oxide, graphene oxide and a combination thereof.
 31. The porous support according to claim 6, wherein the hydrophilic additive is selected from the group consisting of polyhydroxyethylmethacrylate, polyvinylacetate, polyurethane, polydimethylsiloxane, polyimide, polyamide, polyethyleneterephthalate, polymethylmethacrylate, epoxy and a combination thereof.
 32. The porous support according to claim 6, wherein the hydrophilic additive has a mean diameter of 0.005 to 1 μm.
 33. The porous support according to claim 7, wherein the hydrophilic additive is selected from the group consisting of TiO₂ anatase, TiO₂ rutile, TiO₂ brookite, tin dioxide (SnO), zirconium dioxide (ZrO₂), aluminium oxide (Al₂O₃), oxidized single-walled carbon nanotubes, oxidized multiwalled carbon nanotubes, graphite oxide, graphene oxide and a combination thereof.
 34. The porous support according to claim 7, wherein the hydrophilic additive is selected from the group consisting of polyhydroxyethylmethacrylate, polyvinylacetate, polyurethane, polydimethylsiloxane, polyimide, polyamide, polyethyleneterephthalate, polymethylmethacrylate, epoxy and a combination thereof.
 35. The porous support according to claim 7, wherein the hydrophilic additive has a mean diameter of 0.005 to 1 μm. 